1. Field of the Invention
The present invention relates to a radiographic imaging apparatus, method and system. More particularly, the present invention relates to a radiographic imaging apparatus, method and system in which autoexposure control can be performed with precision by monitoring cumulative dose of radiation.
2. Description Related to the Prior Art
An X-ray imaging system is well-known in the medical field in which X-rays are used as radiation. The X-ray imaging system includes an X-ray generating apparatus and an X-ray imaging apparatus. The X-ray generating apparatus generates X-rays. The X-ray imaging apparatus images a body or object receiving X-rays, to form an X-ray image. The X-ray generating apparatus includes an X-ray source and a radiation source controller. The X-ray source emits X-rays toward the body of a patient. The radiation source controller controls operation of the X-ray source. The X-ray imaging apparatus includes a radiographic imaging unit (X-ray imaging unit) such as an electronic cassette, and a console unit. The radiographic imaging unit detects the X-ray image according to the X-rays passed through the body. The console unit controls the radiographic imaging unit, and stores and displays the X-ray image.
The radiographic imaging unit includes a detection panel and a control circuit board. The detection panel, for example, flat panel detector (FPD), detects an X-ray image as electric signal. Pixels are arranged on the detection panel two-dimensionally for storing signal charge according to a dose of X-rays. The control circuit board includes a signal processor and a controller. The signal processor has a switching element and an integrating amplifier. The signal charge is readout from pixels through the switching element, for example, TFT (thin film transistor), stored by the integrating amplifier, and converted into a voltage signal. The signal processor outputs an image signal for constituting the X-ray image.
The controller controls the detection panel in pixel phases of pixel reset, storing and image readout. In the pixel reset, the charge stored in pixels is swept out. In the storing, signal charge is stored in the pixels by turning off the switching element of anyone of the pixels. In the image readout after the storing, the signal charge is read out from a first pixel row to a final pixel row, to output the X-ray image of one frame. In the pixel readout, the signal charge stored in the integrating amplifier is reset (abandoned) at each time of outputting the image signal of one row according to the signal charge, so as to be ready for storing of signal charge of next pixel row.
The pixel reset is to sweep out unwanted stored charge of pixels due to dark current charge irrespective of irradiation of X-rays or residual charge of previous imaging, so as to minimize influence of noise to an X-ray image. The controller drives the detection panel to perform the pixel reset repeatedly before starting irradiation of X-rays. It is necessary in the radiographic imaging unit to synchronize a start of irradiation of X-rays in the X-ray generating apparatus with a start of the storing upon terminating the pixel reset. To this end, a sync signal is transmitted between the X-ray generating apparatus and the radiographic imaging unit. In the radiographic imaging unit, the detection panel is triggered by the sync signal to change over from the pixel reset to the storing.
U.S. Pat. No. 8,536,534 (corresponding to U.S. Pat. Pub. 2012/049,077 and JP-A 2010-052896) and JP-A 2010-075556 disclose an example of the radiographic imaging unit having an AEC device (automatic exposure control device) for automatically controlling exposure of the X-ray image for obtaining appropriate image quality and minimizing exposure of radiation to the body or object. The radiographic imaging unit includes a radiation monitoring device and a dose sampler. The radiation monitoring device detects X-rays incident upon the detection panel. The dose sampler samples a dose signal representing a dose of X-rays per unit time according to an output of the radiation monitoring device. An example of the radiation monitoring device is partial monitor pixels included in the pixels of the detection panel. An example of the dose sampler is a signal processor for reading out the signal charge from the pixels. The dose sampler includes an integrating amplifier, which stores the charge output by the radiation monitoring device according to the dose, and outputs a voltage signal as a dose signal according to the stored charge. The dose signal is sampled at a predetermined sampling period corresponding to a period of storing the charge in the integrating amplifier.
The AEC device acquires a cumulative dose of X-rays according to the dose signal, and checks whether the cumulative dose has become equal to a predetermined target dose. The radiographic imaging unit changes over from the storing to the readout upon the reach of the cumulative dose to the target dose.
U.S. Pat. No. 8,536,534 discloses the radiographic imaging unit having a function of detecting a start of irradiation of X-rays according to a sampled dose signal in the dose sampler, for auxiliary effect in a structure without a communication path to the radiation source controller. In the radiographic imaging unit, dose sampling is started before starting the irradiation of X-rays. The start of the irradiation is detected according to a result of comparison between the sampled dose signal and a predetermined start threshold (turn-on threshold). The storing is started immediately after the start detection.
Also, JP-A 2010-075556 discloses the radiographic imaging unit in which a predicted time point of a reach of the cumulative dose to the target dose is estimated according to a metered dose or the cumulative dose earlier than a reach to the target dose instead of acquiring the cumulative dose until the reach of the cumulative dose to the target dose. An AEC device stops irradiation of X-rays at the predicted time point.
To estimate the predicted time point requires acquisition of changes in the cumulative dose with time. In the radiographic imaging unit of JP-A 2010-075556, the cumulative dose is acquired at two time points during the irradiation of X-rays. The predicted time point is estimated for a reach of the cumulative dose to the target dose by use of linear extrapolation from the cumulative dose of the two time points. In FIG. 13, the radiographic imaging unit acquires a metered dose S11 as the cumulative dose at acquisition time T11 upon lapse of a predetermined period from a reception time T00 of receiving a sync signal from the radiation source controller, according to the sampled dose signal from the dose sampler. Furthermore, a metered dose S12 is acquired at the acquisition time T12 upon lapse of the predetermined period from the acquisition time T11. Then an interpolation line L1 is plotted by linear connection between the metered dose S11 at the acquisition time T11 and the metered dose S12 at the acquisition time T12. An extrapolation line L2 is formed by extension of the interpolation line L1 from the acquisition time T12. A time point at which the extrapolation line L2 reaches the target dose is obtained as the predicted time point.
However, a problem arises with the estimation of the predicted time point TP in the radiographic imaging unit of JP-A 2010-075556. In the X-ray generating apparatus, X-rays are not applied to the radiographic imaging unit at the same time as the radiographic imaging unit receives the sync signal. A time lag occurs from reception time T00 to a start time T0 at which X-rays are applied to the radiographic imaging unit. The time lag differs with various conditions, for example, a fine difference in manual operation of an operator, specificity or specifications of the X-ray source as a product, imaging condition, degradation of the X-ray source with time, and the like. Dosimetry of the cumulative dose in a first event during the time lag is not useful because no X-rays are emitted during the time lag.
Thus, it is necessary to perform the dosimetry of the first event by considering the time lag from the time T0 for the reliability in the dosimetry during irradiation of X-rays after the start time T0. However, the time lag may change with various conditions. The acquisition time T11 must be determined by expecting a longest value of the time lag for safety in all of the various conditions. However, the acquisition time T11 of the dosimetry of the first event is considerably later than the start time T0. At least two time points for the cumulative dose are required for estimating the predicted time point TP. The acquisition time T12 of a second event becomes late in compliance with the delay of the acquisition time T11. A problem arises in considerable waiting time required for obtaining the predicted time point.
Should an interval be too short between the acquisition times T11 and T12, reliability of the estimation of the predicted time point TP may be low. Thus, the interval between the acquisition times T11 and T12 should be sufficiently long. However, the estimation of the predicted time point TP will take long time according to the greatness of the interval between the acquisition times T11 and T12. A serious problem may occur in that the cumulative dose may reach the target dose before terminating the estimating the predicted time point TP so that longer time may pass than is enough for irradiation of X-rays. The body or object is likely to receive unwanted irradiation. The estimation of the predicted time point will be inappropriate for imaging of a low dose with a short irradiation time, because a dose of X-rays is kept low for the imaging.